﻿(3N 
  SOME 
  MOVKMENTS 
  AND 
  REACTIONS 
  OF 
  HYDRA. 
  591 
  

  

  seq.) 
  studied 
  this 
  phenomenon 
  very 
  carefully, 
  and 
  pointed 
  

   out 
  that 
  during 
  such 
  suspension 
  the 
  basal 
  disc 
  of 
  Hydra 
  rests 
  

   at 
  the 
  base 
  of 
  a 
  capillary 
  depression 
  of 
  the 
  surface 
  film. 
  The 
  

   disc, 
  however, 
  is 
  above 
  the 
  surface 
  film 
  and 
  dry. 
  He 
  com- 
  

   pares 
  the 
  suspension 
  with 
  that 
  of 
  a 
  pin 
  or 
  similar 
  object 
  when 
  

   carefully 
  let 
  down 
  into 
  a 
  vessel 
  of 
  water. 
  These 
  observations 
  

   can 
  easily 
  be 
  verified. 
  Scourfield 
  ('01), 
  apparently 
  without 
  

   knowledge 
  of 
  Trembley's 
  work, 
  investigated 
  the 
  same 
  sub- 
  

   ject, 
  and 
  came 
  to 
  practically 
  the 
  same 
  conclusions 
  as 
  Trembley. 
  

   He 
  observed 
  further, 
  however, 
  that 
  there 
  was 
  present 
  on 
  the 
  

   disc, 
  in 
  such 
  cases, 
  a 
  gelatinous 
  substance, 
  often 
  extending 
  

   beyond 
  the 
  disc 
  in 
  minute 
  strands. 
  He 
  believes 
  that 
  this 
  

   substance, 
  as 
  water-repellent, 
  is 
  essential 
  to 
  the 
  process 
  of 
  

   suspension. 
  The 
  fact 
  that, 
  in 
  some 
  cases, 
  he 
  found 
  it 
  difficult 
  

   to 
  force 
  Hydra 
  to 
  leave 
  the 
  surface 
  film 
  adds 
  weight 
  to 
  the 
  

   view. 
  I 
  have 
  myself 
  seen 
  a 
  number 
  of 
  such 
  cases. 
  But 
  

   Trembley's 
  experiment 
  with 
  the 
  pin 
  shows 
  that 
  even 
  without 
  

   such 
  a 
  repellent 
  substance 
  the 
  suspension 
  is 
  easily 
  explained. 
  

   I 
  have 
  observed 
  in 
  a 
  number 
  of 
  other 
  cases 
  a 
  different 
  

   method 
  of 
  suspeusiou. 
  In 
  these 
  a 
  large 
  air-bubble 
  was 
  

   attached 
  to 
  the 
  basal 
  disc, 
  and 
  this 
  bubble 
  apparently 
  kept 
  

   the 
  Hydra 
  afloat. 
  It 
  was, 
  moreover, 
  far 
  from 
  easy 
  to 
  remove 
  

   this 
  bubble 
  or 
  to 
  break 
  it. 
  It 
  seems 
  to 
  be 
  surrounded 
  by 
  a 
  

   tough 
  substance, 
  very 
  probably 
  of 
  the 
  same 
  nature 
  as 
  that 
  

   observed 
  by 
  Scourfield. 
  Suspension 
  from 
  a 
  longer 
  thread, 
  

   such 
  as 
  Scourfield 
  reports, 
  I 
  have 
  never 
  seen. 
  

  

  Reactions 
  to 
  Mechanical 
  Stimuli. 
  

  

  As 
  previously 
  mentioned 
  Hydra 
  viridis 
  contracts 
  and 
  

   expands 
  at 
  much 
  more 
  frequent 
  intervals 
  than 
  the 
  other 
  

   species 
  observed. 
  All 
  its 
  other 
  movements 
  are 
  also 
  more 
  

   prompt 
  and 
  decisive. 
  Hydra 
  grisea 
  and 
  Hydra 
  fusca 
  are 
  

   very 
  sluggish 
  both 
  in 
  responding 
  to 
  stimuli 
  and 
  in 
  recovering 
  

   after 
  their 
  removal. 
  Furthermore, 
  their 
  tentacles 
  expand 
  so 
  

   greatly 
  that 
  they 
  form 
  tangled 
  masses 
  that 
  interfere 
  seriously 
  

   with 
  the 
  accuracy 
  of 
  results. 
  I 
  have 
  therefore 
  used 
  Hydra 
  

  

  